planetary nebula

A planetary nebula is a luminous shell of gas, often of complex structure, cast off and caused
to fluoresce by an evolved star of less than about 4 solar masses. Planetary
nebulae have nothing to do with planets other than their name, which was
coined by William Herschel, in the 1780s,
because their appearance suggested to him the greenish disk of Uranus.

More than 2,000 planetary nebulae have been discovered in the Milky Way
Galaxy out of a total galactic population put at about 10,000.1, 2 This relatively small number reflects the short life span of planetary nebulae
– no more than about 50,000 years. It is a stage through which the
Sun may pass some 5 billion years from now.

Formation

The formation of a planetary nebula begins when a star has evolved to become
a Mira star, a pulsating red
giant that sheds matter in the form of a strong stellar
wind. At this stage, the star has an inactive carbon core that is surrounded
by a helium-burning shell. Instabilities
gradually build up in the outer layers of the star until they break free,
expanding at a speed of about 20 km/s and leaving behind a hot, dead stellar
core.

High-energy ultraviolet radiation pouring from
the exposed core, whose surface temperature is around 100,000 K, is absorbed
by the nebular material and reemitted, mostly in certain unusual spectral
lines, known as forbidden lines,
the brightest of which is the green forbidden line of doubly ionized oxygen
at 5007 Å. The ejected gas shell becomes visible as a glowing disk, ring,
or more elaborate shape.

Infant planetary nebulae sometimes show evidence of a bipolar
flow, as in the case of the Butterfly
Nebula or Ant Nebula. In deep exposures,
the matter ejected at the precursor Mira-variable stage can sometimes also
be detected as an extended halo.

Physical characteristics

Planetary nebulae are typically about 1 light-year in diameter. There is,
however, quite a range of sizes due mainly to the fact that these objects
expand with age. For example, NGC 3918 in Centaurus, which may be only 3,000
years old, measures only 0.3 light-year across whereas the famous Helix
Nebula in Aquarius, which is believed to be at least 10,000 years old,
spans roughly 2.5 light-years.

Although the density of ionized gas in a planetary nebula is higher than
that of the surrounding interstellar medium, it is still extremely low by
everyday standards. Densities range from about 103 particles/cm3 to as much as 106 particles/cm3 in the clumpier parts
of young nebulae. The temperature of the nebular gas averages about 10,000
K, with a range of 8,000 K to 23,000 K, depending on the individual nebula,
the location within the nebula, and the method used to derived the temperature
(for example, observations of He I recombination lines tend to give lower
temperatures than those derived from H I recombination lines).

With a typical expansion speed of 20-30 km/s, the material in a planetary
nebula becomes too spread out to be visible after 10,000-50,000 years.

Morphology

The most widely used scheme to classify planetary nebulae by their appearance
has been the Vorontsov-Velyaminov
scheme. However, in recent years, the use of more powerful instruments,
such as the Hubble Space Telescope, and electronic detection methods have
revealed a much greater diversity in the morphology of planetary nebulae
than was previously realized. To the list of known objects conforming to
the classical ring and disk shapes have been added other, often young planetary
nebulae, with more complex shapes. Roughly one-tenth of planetary nebulae
have a prominent bipolar structure. A few are significantly asymmetric.

Various ideas have been put forward to account for the broad range of morphologies
observed. These include interactions of the nebular gas with magnetic fields
from the central stars,3 interactions between material moving
away from the star at different speeds, multiple ejection events, and, in
the case of strongly bipolar nebulae, gravitational interactions with companion
stars if the central stars are members of binary systems.

Distribution

The majority of planetary nebulae occur near the plane of the Milky Way,
with the greatest concentration near the galactic center, indicating that
they are primarily disk objects. However, a few been found in globular
clusters (for example, Pease 1 in M15) and elsewhere in the galactic
halo.

Some planetary nebulae have also been identified beyond the Milky Way, in
other galaxies of the Local Group, including the Magellanic
Clouds and the Andromeda Galaxy.

Galactic importance of planetary nebulae

In total, planetary nebulae return to the interstellar
medium about 5 solar masses of material each year (about 15 percent
of all the matter expelled by all sorts of stars). This ejected matter,
enriched in elements such as carbon, nitrogen and oxygen, is the stuff from
which a new generation of stars will eventually form.